1. Specialized Sugar Sensing in Diverse Fungi
Victoria Brown, Jeffrey Sabina, Mark Johnston 
Current Biology 
Volume 19, Issue 5, Pages (March 2009)
DOI: /j.cub
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2. Figure 1 Sugar-Sensing Pathways in C. albicans and S. cerevisiae
Glucose signaling begins at the cell surface with the sensors (CaHgt4, or ScSnf3 and ScRgt2) and ends in the nucleus, with deactivation of the Rgt1 transcriptional repressor [1, 11]. The keystone proteins are the transcriptional corepressors (CaStd1, or ScStd1 and ScMth1), which associate with both the sensor and the transcriptional repressor, and it is the levels of these proteins that translate the environmental signal into gene-expression changes. Sugar binding to a sensor activates yeast casein kinase (Yck), which then phosphorylates Std1 and Mth1, thereby marking them for ubiquitylation by the SCFGrr1 complex and dooming them to destruction by the proteasome. Depletion of the corepressors renders Rgt1 impotent, which results in transcriptional derepression of downstream genes. In S. cerevisiae, galactose enters the cell, is phosphorylated, and binds (with ATP) to the Gal3 protein. This complex binds and sequesters Gal80 and relieves the inhibition of the Gal4 transcriptional activator. In C. albicans, CaGal4 does not regulate the GAL genes. Instead, galactose is sensed by the Hgt4 glucose sensor and probably also through Cph1 (a homolog of the S. cerevisiae Ste12).
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3. Figure 2 Hgt4 Regulates Galactose-Induced Genes
Log-phase cultures of wild-type (BWP17), Δhgt4 (CM9 and CM10), or Δhgt12 (CM64) cells were split and incubated in fresh media containing 5% galactose or 5% glycerol at 30°C for 2 hr. Total RNA was reverse transcribed and PCR amplified with primers for HGT7 (orf ), QDR1 (orf19.508), AOX2 (orf ), CMK1 (orf ), HXK2 (orf19.542), and ACT1 (orf ). Control reactions lacking reverse transcriptase yielded no products (not shown).
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4. Figure 3 HGT7 Is Induced in Response to Galactose
(A) The HGT7 promoter was fused to Streptococcus thermophilus lacZ gene, and this construct was integrated into the C. albicans genome at the native HGT7 locus. Cells with HGT7::HGT7-lacZ (CM79 and CM80) were grown in glycerol media, split, and incubated at 30°C for 2 hr in fresh media containing glycerol or 0.04% (top panel) or 1.6% (bottom panel) of the sugars indicated. Cells were lysed and assayed for β-galactosidase activity (in quadruplicate [top] or in triplicate [bottom]), and the results were normalized to the lacZ activity in the glycerol media. Data are presented as mean ± 1 SD.
(B) Cells with HIS1::HGT7-lacZ (HGT4 strains CM230 and CM231 and Δhgt4 strains CM232 and CM233) were grown in media with glycerol as carbon source, split, and incubated at 30°C for 2 hr in fresh media lacking histidine but containing glycerol or the sugar indicated (n = 10 for HGT4; n = 10 for Δhgt4). Black bar: 0.04% glucose; gray bar: 1.6% glucose; striped bar: 0.04% galactose; white bar: 1.6% galactose. All values were normalized to activity in glycerol and expressed as the percentage of the maximum response in 0.04% glucose. Data are presented as mean ± 1 SD.
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5. Figure 4
C. albicans HGT4 Confers a Novel Galactose Response upon S. cerevisiae
(A) S. cerevisiae strains were grown in media containing glycerol, cell densities were normalized, and the culture was split and incubated overnight at 30°C in fresh media containing 5% glycerol or 5% galactose, then lysed, and β-galactosidase activity was assayed (see Supplemental Experimental Procedures). Data are the average of biological duplicates. White bars: wild-type cells + pRS316 vector (YM7642); black bars: wild-type cells +Hgt4-chimera (YM7643); gray bars: Δgal4 cells + pRS316 vector (YM7644); blue bars: Δgal4 cells + Hgt4-chimera (YM7645). Data are presented as mean ± 1 SD.
(B) MTH1 orthologs are galactose-induced in diverse fungi. A phylogenetic tree showing the relationship of yeasts spanning ∼200 million years of evolution is shown [32–35]. Characteristics of the galactose-sensing pathways in these species are described in Table S2. The black circle represents a whole-genome duplication event, the white circle represents the proposed appearance of the Gal4-Gal80 gene-regulatory mechanism, and asterisks indicate the species analyzed in (C).
(C) Each species was grown overnight in glycerol media and incubated in fresh media containing 5% glycerol or 5% galactose at 30°C for 3 hr. RT-PCR was performed on total RNA with the use of species-specific primers for either ACT1 or the MTH1/STD1 ortholog (fungal strains are described in Supplemental Experimental Procedures). First-strand cDNAs served as templates for quantitative PCR. Each MTH1/STD1 signal was normalized to the ACT1 signal in that sample, and the ΔΔCt values are expressed as “Fold Induction” of expression in galactose relative to expression in glycerol (2ΔΔCt). Data are presented as mean ± 1 SD. Separate experiments were performed with the use of semiquantitative PCR for confirmation of the results (see Figure S6).
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6. Figure 5 CaSTD1 and ScSTD1 Function Similarly
(A) CaSTD1 plays a role in the HGT4 pathway. Isogenic strains [HGT4 (CM87) compared to HGT4-1 (CM36) and Δstd1 (CM222) compared to STD1 (CM224)] were grown at 30°C to log phase in media containing glycerol. Cells were harvested and snap frozen, and total RNA was purified for RT-PCR analysis of HGT7 (orf ) or ACT1 (orf ).
(B) CaSTD1 is glucose-induced. C. albicans cells (SC5314) were grown to log phase in media containing glycerol, then incubated at 30°C for 2 hr in fresh media with glycerol (gly) or with the indicated concentrations of glucose (0 indicates no carbon source). Cells were harvested and snap frozen, and total RNA was purified for RT-PCR analysis with the use of primers for CaSTD1 (orf ) or ACT1 (orf ).
Current Biology  , DOI: ( /j.cub )
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